The following are the most likely causes for the PDF file not opening in the WhatsApp program. Detailed explanations can be found here.
PDF document is corrupted -
corrupted PDF documents cannot be opened. Until a corrupted PDF file is restored, it will not open in any application. A mix-up or a specific code in the file format causes the device to become incompatible. It happens when a file isn't downloaded correctly. Alternatively, downloading is interrupted owing to an unforeseeable incident such as a lost or unstable internet connection.
The PDF document is encrypted; to open it, you must either decode it or input a password. If you ignore it or don’t encode the password, the window will be blank or won’t open or it will throw an error saying “file could not be accessed”.